Solid-state imagers are finding increased use in cameras for detecting radiant energy in the visible and infrared light range due to their long life, low power consumption and small size, as compared with conventional image pick-up tubes. Solid-state imagers include an imaging area comprising an array of discrete photosensitive picture elements (pixels) for responding to light from a scene. Typically, solid-state imagers which are suitable for use in television cameras, such as the x-y addressed MOS field-effect transistor type or the self-scanned CTD (charge transfer device) type, have up to 200,000 pixels. Because of random non-uniformities in semiconductor substrate material from which the solid-state imagers are fabricated, and impurities and/or imperfections introduced during the manufacturing process, the manufacturing yield of solid-state imagers having acceptable imaging response characteristics for each pixel decreases substantially as the number of pixels in the imager increases. For example, one type of imaging response characteristic for a solid-state imager is dark current response. It is well known that semiconductor devices exhibit a certain amount of leakage current. In a solid-state imager, the leakage current may result in the collection of a charge in a pixel even in the absence of photo-excitation and is known as the dark current response. When solid-state imagers are used in television cameras, the dark current (nonimage-representative) response of each pixel must be relatively low as compared to its image-representative photoresponse so as to allow television signals to be generated with an acceptable signal-to-noise ratio. However, if the dark current response for a particular pixel is higher than the average level of its surrounding pixels, it will show up as a "white spot" defect in the generated television signal. Alternatively, a pixel can cause a "black spot" defect in a television signal as a result of impurities and/or imperfections introduced during the manufacturing process of the imager. Because of defects like these, the manufacturing yield of solid-state imagers having a large number of pixels, such as those suitable for television cameras, is quite low. Thus, each imager must be carefully tested to screen out those with defects and a high cost is associated with the relatively few imagers which are found to be acceptable.
One way of using such imperfect imagers in a television camera, thereby increasing the number of usable imagers and consequently lowering their cost, is to include a defect corrector in the camera. For example, U.S. Pat. No. 4,179,711, filed in the name of Nagumo, shows a camera wherein a CCD (charge coupled device) imager and a frame memory loaded with defect location information are synchronously clocked. When the defect location memory indicates a signal from a defective pixel is being supplied by the CCD, the signal from a prior pixel is substituted in its place. This type of correction, commonly called "substitution", is generally not desirable for use in television cameras because the substituted signal is clearly visible as being erroneous when viewing a test pattern or a scene having fine detail. Additionally, a large memory is required to store the address of each pixel for identifying which ones are defective, thereby increasing the size, cost and power consumption of the defect corrector.
U.S. Pat. No. 4,200,934, filed in the name of Hofmann, is illustrative of another type of image defect corrector and includes a frame memory for storing the amplitude of the dark current for each pixel of the solid-state imager. The imager and frame memory are then synchronously clocked and dark current amplitudes stored in the frame memory are subtracted from the signals supplied by respective pixels of the imager. This results in an image-representative signal which is substantially free from nonimage dark current variations, as long as each stored dark current amplitude is smaller than the highest possible variation signal for the respective pixel and sufficient signal capacity (headroom) is left for accurately responding to the incident light. This system is advantageous over the substitution method since the correction can be virtually undetectable in a displayed image.
It is known that the amplitude of dark current signals change in accordance with changes in the temperature of the semiconductor material. For example, in CCD imagers dark current amplitudes approximately double for each +8.degree. C. rise in imager temperature. Thus, in the above-described subtraction type defect correction, the temperature of the imager may change between the time when the dark current signals are stored in the memory and the subsequent readout of those signals, leading to inaccurate defect correction.